This invention relates to a process for the pressure leaching of zinc in a two stage leaching process.
In mining operations, such as copper mining involving sulphide ores, waste ore or low grade ore is separated from the higher grade ore which is treated for the extraction of metal, such as copper, therefrom.
The low grade or waste ore is also treated for copper recovery, e.g. by leaching the ore in the waste dump (dump leach) to produce a copper solution, from which the copper is extracted to produce a raffinate.
Due to the high pyrite content of the waste ore and natural oxidation of this ore by atmosphere oxygen, aided by natural bacteria, the iron concentration of the raffinate resulting from the leaching of this ore builds up over the many years of operation.
Attempts have been made to remove the iron from the solution, without success, resulting in large amounts of acidic iron containing solution being accumulated.
It is accordingly an object of the present invention to provide a method whereby the iron can be removed from these solutions.
U.S. Pat. No. 4,004,991 describes a process for the pressure leaching of zinc and iron containing mineral sulphides in dilute H2SO4 solution. The pressure leaching is carried out in a two-stage countercurrent process in which finely divided sulphides are leached in a first leaching stage with solution from the second leaching stage to produce a first stage leach solution containing a high zinc concentration and low iron and H2SO4 concentrations. The first stage leach solution is subjected to purification treatment and then treated for zinc recovery by electrowinning. The residue from the first stage is pressure leached in the second stage with return electrolyte from the electrowinning process.
It is another object of this invention to provide an alternative zinc extraction process in which the iron bearing raffinate from waste dump leach can be used.
According to the invention there is provided a process for the recovery of zinc from a zinc sulphide ore or concentrate, comprising steps of subjecting the concentrate to pressure leaching with an acidic leach solution containing at least about 15 g/L iron in solution to produce a zinc solution and a solid leach residue containing iron; subjecting the zinc solution to zinc solvent extraction to produce a raffinate and a pregnant zinc solution; and subjecting the pregnant zinc solution to electrowinning to recover zinc and producing a resultant spent electrolyte.
The acidic leach solution may contain 15 g/L or more iron and about 15 to 20 g/L free acid or about 50 g/L sulphate total, combined sulphates of iron and sulphuric acid.
Also according to the invention there is provided a method of removing iron from an iron bearing raffinate comprising the steps of pressure leaching a zinc sulphide ore with said iron bearing raffinate to produce a solid leach residue containing iron and a zinc solution.
The pressure leaching is preferrably carried out in two stages countercurrently, comprising a first stage leach to produce the zinc solution and an intermediate residue; a second stage leach wherein the intermediate residue is leached with said acidic leach solution containing at least 15 g/L iron to produce said leach residue containing iron and a partly exhausted iron containing leach solution; and effecting the first stage leach with said partly exhausted ion containing leach solution.
Preferably, the acid leach solution contains at least about 30 g/L iron.
Further objects and advantages of the invention will become apparent from the description of a preferred embodiment of the invention below.